The present invention relates to an active heater control that is used in connection with aerospace probes that sense environmental parameters, such as pitot or pitot-static tubes, fuselage mounted total air temperature sensors, and engine inlet air temperature sensors, which all employ heaters to keep the probes free of icing during aircraft operation. The heaters usually are on continuously in flight, and during selected periods of time on the ground. The active heater control circuit of the present invention reduces heater power during conditions of low mass air flow thereby avoiding heater or probe damage or other performance problems.
In the prior art, traditional deicing heaters on probes, such as pitot, pitot-static, total air temperature and engine inlet temperature sensor probes employ nickel-based resistors that rely on the rise in electrical resistance with temperature exhibited by nickel to reduce current load. Heaters of this type provide only rudimentary temperature control when power is applied in the form of a constant voltage because the change in resistance of nickel with temperature is not large. As a consequence, probe temperatures often become excessive when convection cooling is poor, that is, when the air mass moving over the probe is low, such as when an aircraft is on the ground, either stationary or taxiing. Excessive probe temperatures can result at that time, causing reduced reliability of the heating element. Excessive probe temperatures also cause severe thermal stresses in the sensor assembly leading to various failure modes. In the case of total air temperature probes, excessive temperatures increase errors in the total air temperature measurement, referred to as the deicing heater error or DHE.
Active heater controllers (AHCs) using semiconductor switches such as power MOSFETs, transistors, triacs, and SCRs are often utilized as switches to control power to resistive heating elements. A major impediment to the use of such active devices on aerospace probes is their incompatibility with existing aircraft systems. The problem lies with the aircraft current monitoring circuitry which expects relatively high, substantially continuous current levels typical of existing probes. An AHC will pulse the current in the form of regular on/off cycles in order to achieve temperature control. On/off cycling is desirable because it reduces power dissipation in the active device. It is preferable to have the active device, or switch, either fully xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d as opposed to an intermediate state, which forces the active device to dissipate power in order to control heater element current to a desired level.
Active temperature control is achieved by controlling the xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d periods of the active device, allowing the thermal inertia of the probe to smooth out the final probe temperature. To maintain reasonable temperature control, the xe2x80x9coffxe2x80x9d period often becomes too long, sometimes on the order of seconds. Aircraft current monitors presently used interpret these dormant or off periods, (i.e., when the current draw is zero), as a probe heater failure. The indication of failure is unacceptable and has prevented active heat controller usage on existing aircraft without modifying the aircraft""s current monitoring circuitry, a costly and impractical task for aircraft already in service.
An active heater control according to the present invention controls a probe or sensor heater so that the time that the heater is on when there is low mass flow past the probe is short enough to avoid heater damage and other problems with thermal stresses in the probe or sensor assembly while being compatible with existing aircraft current monitoring circuitry. False signaling of unintended heater failures is avoided by providing an xe2x80x9coffxe2x80x9d period of the active device that is less than the time constant interval of the aircraft""s current monitoring circuit.
Once the maximum xe2x80x9coffxe2x80x9d period of the aircraft current monitor before signaling a failure is established or known, the active heater control circuitry can be modified to generate a series of override pulses which effectively force the active switch to turn on regardless of the probe housing temperature. However, during the xe2x80x9coffxe2x80x9d period of the override, or in other words, at times when there is no override pulse, temperature control operations will proceed as normal. The end result is the controller will always be on for a predetermined minimum time at a rate constant enough to assure the aircraft current monitor does not signal a probe heater failure, when none has occurred.
Stated another way, the present invention provides pulses that will activate the heater on the aircraft probe at periods of time that will insure the aircraft""s current monitoring circuitry will sense that the heater is functioning, without over heating the probe itself. For example, if a probe is off, for example, 4 seconds, the aircraft current monitoring circuitry would indicate that the probe heater was inoperative or had failed, and would provide a signal indicating failure. If the probe is on for a short period of time at periods less than the time for the current monitor to signal failure, that is, for example, every three seconds, the aircraft current monitor would recognize that the heater was functioning.